The invention uses retro-reflected light having a fixed component and a variable component varied by a person, from a retro reflector connected to a person to input data to an information system and does this for each of several persons in the same time interval.
Limits of data input via retro-reflected light—for example by Honey in U.S. Pat. No. 5,912,700 and by Stuart in U.S. Pat. No. 4,998,441—are surmounted by the discovery shown here.